This invention relates to electrodischarge machining and, more particularly, to a technique for electrodischarge machining workpieces with holes therein without plugging the holes.
In electrodischarge machining, often abbreviated as xe2x80x9cEDMxe2x80x9d, an electrode is positioned adjacent to a machining location of a workpiece, with a small gap therebetween. An electrical current is passed between the workpiece and the electrode, and material is removed from the workpiece by a sparking discharge action. The machining location and the portion of the electrode adjacent to the machining location are immersed in a liquid dielectric electrodischarge medium. The liquid dielectric electrodischarge medium occupies the gap and aids in removing the residual material machined from the workpiece and cooling the workpiece. A typical liquid dielectric medium is an oil with a vegetable oil base.
Electrodischarge machining works well in many applications. In others, problems have been encountered. The inventors are interested in performing electrodischarge machining of weld-repair deposits at the tips of gas turbine blades. The gas turbine blades have holes for cooling air in the sides of the airfoil section. When these turbine blades are electrodischarge machined, the liquid medium leaks into the cooling holes of the turbine blade. The inventors have observed that, after the electrodischarge machining is complete, the liquid medium in the cooling holes is removed only with great difficulty. If complete removal is not effected, the residual liquid medium chemically reacts with chemicals present in the subsequent processing operations, such as detergents, and results in plugging of the cooling holes during further processing operations.
There has been previously proposed no approach for solving this problem. Masking of the cooling holes may allow reactive chemicals to remain, leading to their plugging in later steps of the processing. The use of many effective cleaning chemicals, such as trichloroethane, is not acceptable environmentally and may pose a health hazard. Accordingly, there is a need for an approach to electrodischarge machining in which small holes or openings in the workpiece are not plugged during or after the electrodischarge machining. The present invention fulfills this need, and further provides related advantages.
The present invention provides an electrodischarge machine method and apparatus which is operable for electrodischarge machining but which overcomes the problem of the plugging of holes in the workpiece. The present approach may be implemented as a modification to conventional electrodischarge machines. The present approach yields excellent results, yet is environmentally friendly and safe.
A method for electrodischarge machining a workpiece comprises the steps of furnishing a machining apparatus comprising a workpiece holder, an electrode holder having an electrode mounted therein, and a power supply providing an electrical voltage and current. The workpiece is mounted in the workpiece holder, and the electrode is positioned adjacent to a machining location of the workpiece. The method further includes furnishing a source of a flow of conditioned water, directing the flow of conditioned water from the source into the machining location of the workpiece, without submerging (immersing) the machining location in a liquid, and applying a voltage from the power supply such that an electrical current flows between the workpiece and the electrode holder and metal is removed from the workpiece into the flow of conditioned water. The workpiece is not contacted by an oil during electrodischarge machining and is not contacted by a detergent during the post processing and cleaning steps.
Preferably, the electrode is positioned vertically above the workpiece. The step of directing the flow of conditioned water includes the step of directing the flow of conditioned water over a surface of the electrode so that the conditioned water flows downwardly over the machining location and a surface of the workpiece. To introduce the conditioned water, a plurality of water flow orifices may be provided in the electrode holder and positioned to direct the flow of water against a surface of the electrode.
The conditioned water used in the present approach is a partially deionized water having an electrical resistance of from about 10,000 to about 150,000 ohms per square centimeter, preferably from about 10,000 to about 100,000 ohms per square centimeter, and most preferably from about 10,000 to about 15,000 ohms per square centimeter.
The source of conditioned water desirably comprises a water flow orifice positioned to deliver conditioned water to the machining location, a reservoir of conditioned water, preferably in the form of a non-metallic catch basin, positioned to catch water that drips from the workpiece, and a pump that receives the water from the reservoir and delivers pressurized water to the water flow orifice. The water may be filtered and re-conditioned to remove bits of the material removed from the workpiece and any ionic components that are present, to restore it to its conditioned state prior to reuse by flow through the water flow orifice.
In a preferred application, the workpiece is a gas turbine blade, particularly a gas turbine blade having a cooling opening in an airfoil surface thereof. The machining location is a tip of a gas turbine blade, such as a tip to which a weld repair has been applied and which is to be electrodischarge machined to final form.
The present approach uses conditioned water as a medium for the electrodischarge machining. The workpiece and its machining location are not submerged in the conditioned water, but instead the conditioned water is continuously flowed over the surface of the workpiece and the machining location. The conditioned water provides a dielectric medium and also removes the machining remnants and heat from the vicinity of the machining location. There is nothing in this medium to foul the cooling holes in the workpiece in the case of a cooled- gas turbine blade. Accordingly, there is no need for masking or other protection applied to the gas turbine blade, or for any complex cleaning procedures following the electrodischarge machining. The electrodischarge machining of this approach is accomplished at rates comparable with those achieved in convention electrodischarge machining, with lesser adverse side effects.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.